This invention pertains generally to the field of electrical power conversion systems and particularly to power converters for supplying power to single-phase induction motors.
Single-phase induction motors typically include a main stator winding which provides the main drive power for the motor and an auxiliary winding which is supplied with power through a capacitor to provide starting torque for the motor. The auxiliary winding may be disconnected when the motor reaches operating speed, as in a capacitor start motor, or it may be supplied with power constantly as in a permanent split capacitor motor. The capacitor connected in series with the auxiliary winding provides the phase shift of the voltage applied to the auxiliary winding that allows starting torque to be developed. Such single-phase induction motors are widely used in residential and commercial applications, such as in home appliances, fans, pumps, etc., and are manufactured in large volumes at relatively low unit cost.
Single-phase induction motors inherently operate at a constant speed determined by the AC power line frequency (50 or 60 Hz) and the load imposed on the motor. For various applications, such as fan drives, it would be desirable to be able to operate the motor at selectable speeds. The operating speed of a fan drive motor can be reduced by simply reducing the AC voltage applied to the motor. While this approach allows the use of inexpensive control circuitry, operating the motor at reduced voltage levels lowers the energy efficiency of the motor and generally does not result in close control of the motor speed. Another approach is the use of a variable frequency converter which receives AC power from the AC power system, rectifies it, and inverts the power to an approximately sinusoidal output voltage at a selected frequency that may be higher or lower than the normal line frequency. While such power conversion systems can provide effective variable speed drive of the motor, the cost of the power converter itself is significant (particularly as compared to the low-cost induction machine it is driving) because of the complexity of the power converter and the need for several (typically at least six) semiconductor switches which must be rated to handle the maximum voltage and current to be supplied to the motor. Consequently, such power converters have not been practical for use in low-cost applications such as single phase fan motor drives.
In accordance with the present invention, an adjustable speed drive is provided having a simple, low-cost structure that is well suited for use with low-cost single-phase induction motors. The drive is capable of operating in a full-speed mode with high starting torque, and in at least one lower speed mode which is well suited for applications such as fan motor drives. The adjustable speed drive of the invention may also be operated in multiple discrete speed modes or continuously variable speed modes if desired. The adjustable speed drive of the invention supplies power to the motor at higher efficiencies than can be obtained with conventional low-cost drives that reduce the voltage applied to the motor to reduce speed.
The adjustable speed drive of the invention includes a rectifier which receives AC power at first and second AC input lines from the AC power mains and provides DC power across DC bus lines, and a single-phase two switch inverter which provides AC power at an output line at a selected frequency and phase shift with respect to the input power from the main power lines. The output of the inverter is supplied to a transfer switch circuit which is also connected to the second of the input lines to receive AC power from the AC power lines. The transfer switch circuit also is connected to a phase-shifting capacitor and has a main winding supply line connectable to the main winding of the motor and an auxiliary winding supply line connectable to the auxiliary winding of the motor. Both the main and auxiliary windings are connected to the first of the AC input lines to complete the circuits through these windings. For full-speed mode operation, the inverter is operated to provide an output voltage that is at line frequency and approximately 90xc2x0 out of phase with the line voltage. The transfer switch circuit connects the inverter output to the auxiliary winding and connects the second input line to the main motor winding. Operation in this mode provides relatively high starting torque, superior to that obtainable by the use of a capacitor to provide the phase shift for the voltage applied to the auxiliary winding. After start-up, the inverter may continue to provide power to the auxiliary winding for maximum efficiency. The torque capability of the motor is improved by the fact that a voltage of correct amplitude and 90xc2x0 phase shift is provided by the inverter, allowing operation over a wider range of loads than is available with a capacitor-run motor. Operation may also be in a mode analogous to the operation of a capacitor start motor, in which the inverter may be disconnected or turned off so that power is supplied only to the main winding through the transfer switch circuit after the motor reaches operating speed as determined by the motor load and line frequency (e.g., 60 Hz). For operation at reduced speed, for example, for fan drives for heating, ventilating and air conditioning systems, the inverter may be operated to provide AC output power at a frequency lower than line frequency (e.g., 30 Hz for approximately half-speed operation), and the transfer switch circuit is operated to supply power from the inverter output directly to the main winding of the motor and through the capacitor to the auxiliary winding of the motor, thereby providing phase-shifted voltage (at the lower frequency) to the auxiliary winding to provide start-up torque. Power may also be provided from the inverter through the capacitor to the auxiliary winding after the motor is up to speed. The inverter may also be operated at additional discrete or continuously variable frequencies to further expand adjustable speed drive control of the induction motor in this mode.
In a preferred adjustable speed drive in accordance with the invention, the rectifier includes a pair of rectifying diodes connected together at a node and connected across DC bus lines. A pair of energy storage capacitors is also connected across the DC bus lines, with a node between the capacitors connected to a first of the input lines, and with the second of the input lines connected to the node between the pair of diodes. The diodes and capacitors serve to rectify AC voltage from the main power system to a DC voltage across the DC bus lines. An inverter comprised of two gate controlled switching devices is connected across the DC bus lines. A first output line is connected to the first input line, a second output line is connected from the second input line to the transfer switch circuit, and a third output line is connected from the node connecting the pair of controlled switching devices to the transfer switch circuit. The first output line is connectable to both the main winding and the auxiliary winding of a motor to be driven. A main winding supply line extends from the transfer switch circuit for connection to the main winding and an auxiliary winding supply line extends from the transfer switch circuit for connection to the auxiliary winding. A phase-shifting capacitor is connected to the transfer switch circuit. The transfer switch circuit can comprise a first switch connected on one side to the second and third output lines and on the other side to the main winding supply line, the first switch switchable between a position connecting the second output line to the main winding supply line and a position connecting the third output line to the main winding supply line. The transfer switch circuit can further comprise a second switch connected on one side directly to the third output line and to the third output line through a phase-shift capacitor and with the auxiliary winding supply line connected on the other side of the second switch. The second switch is switchable between a position connecting the third output line to the auxiliary winding supply line directly and a position connecting the third output line to the auxiliary winding supply line through the phase-shift capacitor. The first and second switches may be switched together and may comprise a relay, in the first position of which the second output line is connected directly to the main winding supply line to supply power directly from the power system to the main winding and the third output line is connected to provide power from the inverter through the auxiliary winding supply line to the auxiliary winding. In a second position of the transfer switch, the third output line is connected directly from the inverter to the main winding supply line to supply power to the main winding and the third output line is connected through the phase-shift capacitor to the auxiliary supply line to supply to the auxiliary winding power that is phase-shifted from the power applied to the main winding.
A controller is connected to the switching devices of the inverter and to the transfer switch circuit to respond to input commands to change the two positions of the transfer switch circuit and to appropriately control the switching of the switching devices in the inverter to generate AC output power at the appropriate frequency and phase shift. The circuit constructed in this manner can be utilized to operate at discrete frequencies and thus discrete motor speeds as well as allowing continuously variable driving of the motor, if desired, utilizing a minimum number of components. The switching devices of the inverter, e.g., semiconductor switches such as IGBTs or MOSFETs, can be devices which have a significantly lower rating, and thus lower cost, than devices which would be required to drive the motor at full speed. When the inverter is utilized to provide power to the auxiliary winding during full-speed mode operation, the current drawn by the auxiliary winding is significantly less than that drawn by the main winding, which is supplied directly from the power lines through the transfer switch. In the lower speed (and lower frequency) mode or modes, the torque-producing currents drawn by the main winding and auxiliary winding are generally significantly less than currents drawn at full speed. For example, the power required to drive a fan increases as a cubic function. Thus, the power required to drive the fan at half-speed is only xe2x85x9 that at full speed, and an inverter designed to operate in a second mode at half-speed may utilize switching devices which are rated for only xe2x85x9 of the full speed motor power.
Although less torque is required due to the typical load characteristics at lower speed and lower excitation frequency, the magnetic flux levels in the motor will increase as excitation frequency decreases if winding voltage remains constant. To avoid saturation, the invention can be adapted to reduce inverter output voltage to maintain a fixed direct proportionality relationship between winding voltage and excitation frequency. This is familiar to those skilled in the art as xe2x80x9cconstant volts per hertzxe2x80x9d control.